Transmission of light through a medium, such as blood, consists of having a light emitter give off light, and the light then passes through the medium and is then incident on a photodetector. The emitter may be at least one light emitting diode (LEDs), laser, laser diode, or other device, and the photodetector could be a photodiode or other detection device.
An array of LEDs may serve as emitters and a matching photodiode may be used for detection. The term matching means that the photodetector has electrical characteristics such that the discrete wavelengths emitted by the array of LEDs produce a current in the photodetector. The range of wavelengths over which the photodetector can produce a current is termed the photodetector bandwidth. The photodetector currents can be amplified and converted to voltages through the use of a gain stage termed a Trans-impedance Amplifier (Trans-Z Amplifier).